Syria's former prime minister Riyad Hijab claims Bashar al-Asssad's regime is on the point of collapse, having lost control of two-thirds of the country.

In his first public appearance since he fled Damascus with his family a week ago, Hijab urged other top officials to follow his example and defect. He told a press conference in the Jordanian capital, Amman, the Syrian army needed to "take the side of the people".

"I assure you, from my experience and former position, that the regime is collapsing, spiritually and financially, as it escalates militarily," Hijab said. "It no longer controls more than 30% of Syrian territory."

Hijab said that while he was prime minister he had been unable to stop the regime using heavy artillery against Syrian cities it considered opposition strongholds. He said he had felt "pain in my soul" at the shelling of civilian areas.

"I was powerless to stop the injustice," he said. But he added: "Syria is full of honourable officials and military leaders who are waiting for the chance to join the revolution. I urge the army to follow the example of Egypt's and Tunisia's armies and take the side of people."

In Damascus, the regime shrugged off Hijab's defection and his new claims. Kadri Jamil, one of four deputy prime ministers in the Syrian government, told the Guardian that Hijab's defection had been a surprise and that here had been no inkling from the views he expressed in cabinet that he was about to become a critic.

"He was a good actor. He wasn't truthful with us. He was a double-faced person," Jamil said, arguing that such defections could have a positive impact for the government.

"There's a process of natural selection going on within the regime. It should have started long ago. The regime didn't have the courage to do it, but events have done it. All the corrupt elements within the regime are leaving the ship because they think it's sinking."

He also hinted that foreign intelligence services had helped Hijab to defect.

Opposition figures claim many other leading military and political figures in the regime have swapped sides but remained at their posts, either out of fear of what would happen to their families if they defected, or because they had been asked to stay by the rebels to supply intelligence on the inner workings of Assad's government.

Hijab is the highest-ranking defector to date. His flight from Damascus came a month after the defection of a Republican Guard general and former member of Assad's inner circle, Manaf Tlass, and the Syrian ambassador to Baghdad, Nawaf al-Fares.

In the past week the head of protocol at the presidential palace, Muhi al-Din Maslaman, has also defected.